Dynamical Phase Transitions and the Glass Transition

The transition from liquid to glass a very challenging open problem in physics. While we know that for liquid like water a genuine equilibrium phase transition takes place as it evaporates, yet there is no consensus on whether for glasses a true equilibrium phase change occurs at low temperatures. Gaining a better understanding of glasses is complex because very low temperature equilibrium liquids are vey hard to access, both in experiments and computer simulations: the viscosity is so large that the system is not ergodic, and falls off equilibrium.


In order to overcome these difficulties, we use clever numerical algorithms that combines Transition Path Sampling and Umbrella Sampling [see here or here]. This allows us to drive model atomistic glass former off-equilibrium in a controlled manner in order to gather normally inaccessible equilibrium thermodynamic information about its very low temperature behaviour. This proves the existence of an exceptionally low energy disordered state and highlight that it coexists with the normal liquid in a wide range of temperature. Intriguingly, we find that the two states become undistinguishable at temperatures at which the viscosity is so large that the system behaves mechanically as a solid.

These findings open the way to a novel interpretation of the anomalies in supercooled liquids and glasses (large dynamical fluctuations, emergent solidity) helping to unify viewpoints different pictures of the phenomenon.

Active matter and Surface Phase Transitions

Many classes of active matter models show aggregation and eventually motility induced phase separation (MIPS) in the bulk. Even more challenging than the bulk properties is the surface physics of active systems. For example, the very notion of surface tension is controversial and it is not clear what kind of fundamental principles may hold: is there any equivalent to the equipartition of modes that is essential for capillary waves in passive systems? what is the relation between surface stiffness and surface tension? why, ultimately, are active interfaces stable? These questions open an avenue to the study of surface phase transitions in active matter: in passive systems, the presence of a liquid-gas phase separation is a pre-requisite for wetting and drying transitions. Inspired by the analogy between equilibrium binodals and MIPS, I am currently studying whether wetting and drying have any corresponding form in the active case and under what conditions one can promote them.

Active Brownian particles moving in presence of a finite energy barrier in the middle of the simulation box. The colours show regions of high (red) and low (blue) density. The low density region is localised around the barrier for a longer time than the high density region.

Physical gels are disordered solid formed when a liquid undergoing a gas-liquid phase separation is locked into an arrested state at low temperature. Very strong, short ranged attractive interactions cause the constituents to locally collapse onto compex, branched structures that span over very large lengthscales, giving rise to materials that are halfway between a solid (elastic response) and a liquid (flow).

The formation of a percolating cluster in a very dilute gel (see here).

With computer simulations, one can explore model gels that resemble the ones that can be procuded in colloidal experiments. If, on the one hand, simulations are limited by the very long time that takes for a gel to relax (which is often computationally challenging) on the other hand they contain a lot of detailed information, concerning the microstructure, the forces and the dynamical correlations in the gel network. Thanks to thisinformation we can understand from a microscopic point of view th emergence of the macroscopic properties, such as the solid-like resposne of gels.

Interface Growth Kinetics

It is of paramount importance for materials-science  to study the time evolution of different phases (liquid, amorphous or crystalline solid) for meso and macroscopic systems. This is often modeled with coarse-grained theories such as phase field, phase field crystal or, at a more fundamental level, dynamic density functional theory (DDFT). Yet, the details of the dynamics and the kinetic processes that trigger the progression of one phase at the expenses of another can be resolved in an accurate manner by the means of molecular dynamics, which unfortunately is intimately bounded to the exploration of relatively small length and short time-scales. Determining which assumptions of the coarse-grained theories can be motivated from a microscopic point of view  is therefore of fundamental importance. To do so, we have focused on the non-equilibrium dynamics of  liquid-solid interfaces, studying simple but yet realistic model systems of crystal growth by the means of molecular dynamics, with the perspective of a thorough comparison with  the predictions of several alternative DDFT models.

Short polymers crystal nucleation and growth
Interested in the still open questions of the kinetics of polymers crystallization, I have been studying the dynamics of short flexible chains. We have analyzed the formation of the critical nucleus and the crystal growth process in a model system of eicosane. We have determined via committor analysis the characteristic nucleus size and we have shown that the chains that form the critical nucleus first align, then straighten, and finally the local crystal structure forms.

The growth of the crystal advances mainly through a sliding-in process on the lateral surface, which takes place in a correlated way, i.e. chains tend to get attached in clusters.

Large deviations and kinetically constrained models

During the PhD I have mainly worked on transport problems and their study via numerical methods.

In particular, I have considered exclusion processes and kinetically constrained models in order to explore some recent advances in modern statistical mechanics.

Senza titolo

The study of the large deviation functions of dynamical observables applied to these models allows to show a dynamical phase transition related to the jamming and glass transition problem. Tools from the thermodynamics of histories allow to spot heterogeneities in the driven flow of kinetically constrained particles. See my PhD thesis.

Biologically inspired asymmetric exclusion processes
Senza titolo

Biological transport processes in cells take place on substrates that  are often coupled to the active motion of macromolecular complexes, such as motor proteins on microtubules or ribosomes on mRNAs. Inspired by biological processes such as protein synthesis by ribosomes and motor protein transport, we have discussed the concept of localized dynamical sites coupled to a driven lattice gas dynamics. We investigated the phenomenology of transport in the presence of dynamical defects and found a regime characterized by an intermittent current and subject to severe finite-size effects. Our results demonstrate the impact of the regulatory role of the dynamical defects in transport not only in biology but also in more general contexts. 

City traffic by simple models

During my master degree, I have been working at the modelling of the traffic flow fundamental graph in a town. See my master thesis (in Italian).

Go to my publications.